CVD reactors are utilized for vapor depositing coatings on some type of flat surface. Reactors of this type are especially useful in the semiconductor industry for vapor depositing coatings on semiconductor wafers. In one very specific application metal organic chemical vapor deposition (MOCVD) reactors are utilized to deposit compound semiconductor epitaxial layers on semiconductor wafers, such as GaAs. When depositing GaAs epitaxial layers it is extremely important that the layers have a uniform thickness. Further, it is much easier to achieve acceptable uniformity in smaller semiconductor wafers having a diameter of 2-3 inches, whereas uniformity becomes a very real problem when the size of the wafers is increased to a diameter in the range of 4-8 inches.
In a horizontal CVD reactor, the reactant is depleted in the flow direction as it flows over a susceptor and the wafer to be coated. The growth rate is: EQU g(r)=g.sub.o *exp-A*[Kg*r/(h*v)]
where:
h is the stagnant layer thickness, PA1 kg is a reaction coefficient, PA1 A is an experimentally determined constant, PA1 v is the gas velocity.
One way to improve uniformity of deposited layers is to increase the gas velocity by using high gas flow and/or low reactor pressure. Usually this is at the expense of low material utilization efficiency. Another way which has been commonly used in the past to improve the uniformity, however in somewhat of an arbitrary fashion, is to rotate the substrate as the material is being deposited. The ideal is to average out the gas depletion effect by rotating the substrate. For a substrate of small to moderate size, i.e. 1-3 inches, this method seems to be satisfactory to a certain extent, e.g. for a typical uniformity of no less than .+-.5%. However, for larger wafers, i.e. 4-8 inches and above, an averaging scheme by rotating the substrate alone is not sufficient to achieve stringent uniformity requirements, e.g. .+-.1% or less, since the growth rate curve is integrated with time in a nonlinear fashion (cosine-like). The thickness at the center of the wafer after one rotation is a product of a constant growth rate at that position and one cycle time. On the other hand, the thickness at the edge of the wafer after one rotation is proportional to the integration of the growth rate profile with time.